Diphtheria is an acute, contagious disease caused by the Corynebacterium diphtheriae. It is spread through respiratory droplets of infected individuals. The bacteria, lodging in the mucous membranes of the throat, secrete a potent toxin, which causes tissue destruction and the formation of a gray membrane in the upper respiratory tract that can loosen and cause asphyxiation. The toxin may also spread via the blood and damage tissues elsewhere in the body.